Many different tapping attachments have been utilized on multiple spindle automatic machine tools. The early units were typically such as those shown in U.S. Pat. No. 1,130,398, with the tap holder in the tooling area, and much of the mechanism in the gear box. This mechanism included a cam for longitudinal feed of the tap holder plus different gears for the advance and retract movements plus clutches to establish the advance and retract movements. Other manufacturers of multiple spindle machine tools also generally followed this practice of cam-driven movement and gears in the gear box, as shown in U.S. Pat. Nos. 2,161,548 and 2,236,440.
Some efforts were made to make tooling attachments which were more accessible in the tooling area, such as shown in U.S. Pat. No. 2,268,944, but these were still driven by cams and required two different motors and a differential to obtain the threading and unthreading motions.
Another attempt at a threading attachment which was primarily in the tooling area is shown in U.S. Pat. No. 2,376,164, which had a motor which could be speeded up for a fast traverse of the tap toward the workpiece and then slow speed for tapping, but the motor was reversed for tap withdrawal and ran at the same speed in reverse, and it also required a differential mechanism. U.S. Pat. No. 3,802,298 showed a unit head for a multiple station machine tool which had a feed motor and a separate traverse motor, plus a brake.
The typical threading attachment, such as shown in U.S. Pat. Nos. 2,161,548 and 2,236,440, had considerable mass, friction and inertia, so that with very small taps in a workpiece, the frictional drag was apparent by shaving on the front or rear flanks of the threads cut in the workpiece. Also, such threading attachments generally fit in only two of the perhaps six positions of the spindles of the multiple spindle machine, thus limiting their versatility. Still further, the typical threading units had only three tapping speeds for advance movement and two speeds for backing the tap out of the work, and this was for right-hand threads. If it was desired to make left-hand threads, then there were only two tapping speeds and three speeds for backing the tap out of the work. All of these meant difficult gear changes in a rather inaccessible place, usually within the gear box. In these prior art units, one merely chose the best gear ratios available, and this was not necessarily the best choice for the type of material and diameter of the tap.